1. Field of the Invention
The present invention relates to a multi-stand mandrel-free stretch reducing mill for reducing an external diameter of a tube. The stretch reducing mill includes a roll pass for roll sets, each of which has three rolls. The roll pass has a roll pass shape that deviates from the circular, takes into account the modification of the shape of a cross-section of a tube, and counteracts a non-circular external diameter of the tube.
2. Description of the Related Art
It is known in the prior art in stretch reducing mills to use roll pass shapes that have contours deviating from the circular. These deviations have different purposes, e.g., to prevent material from entering a gap between rolls and causing surface damage to a rolled material. The deviations are also meant to minimize wear on the rolls in the region of roll flanks.
Moreover, roll pass shapes deviating from circular are used, depending on diameter reduction and wall thickness/diameter ratio of the rolled material, to avoid the formation of irregular shapes in an interior of the rolled material. In practice, to avoid this internal polygon formation, as it is called, non-circular roll passes with a wide variety of roll pass shapes have been proposed, as described above.
In contrast, exactly round rolls have generally been used until now for finish-rolling of tubes in a final finishing pass or passes of the stretch reducing mill (wherein an amount of heat-related shrinkage is taken into account as needed). However, it has been found that such finish-rolled tubes, upon leaving the stretch reducing mill, have external diameter fluctuations of up to 0.3% to 0.5% of their average diameter. These fluctuations occur due to non-uniform modification of the shape of a cross-section of the rolled material.